The manufacture of cigarette filters, or related products, that have a tubular structure is well-known in the art. These typically rely on the thermoforming of a plasticised tow of cellulose acetate around a shaped die to form a continuous tube, which is then cut into finite lengths. Other variants are known in which the fibre tow is formed around a moving mandrel to form a void within the filter material; the material can then be cut in the void region to create a filter tip which is in the shape of a tube at the mouth end but is closed at the other end. The above methods may be used to form cigarette filters and filter elements that have an internal channel or bore (or a number of smaller channels or bores) extending from, and exposed at, the mouth end. The internal channel or bore may be circular in cross section, or may have a cross section of another defined shape (e.g. triangular, star-shaped, heart-shaped, etc.). The cross section of the channel is immediately visible at the mouth end and presents a distinctive image to the smoker, and therefore the monitoring and control of the visual appearance provided by the channel is highly important. The cross section of the channel can become distorted, ill-formed or ill-defined (e.g. loose fibres can give a ‘hairy’ end appearance to the channel), and it is vital that product filters/element with such misshapen channels do not find their way to the customer, and also that production is stopped so any fault in production (which is leading to misshapen product) may be quickly rectified.
To date, there is not a simple automated technique available to monitor the visual properties of the end of tubular shaped filters. Instead, visual quality checks would be routinely carried out by trained personnel after filter production. Clearly, this is time-consuming, expensive and open to interpretation, depending on the operator concerned. Further, if the fault is common to a large number of product filters, it may be due to a problem in the manufacturing process; monitoring product quality after the production run does not allow for halting production if there is a problem with the process. There is thus a need for a visual inspection system for the ends of tubular, shaped and other filters that is rapid, accurate and reliable, and which can easily be integrated “in line”, as part of a high speed filter production process. Although vision systems are known and used for monitoring quality parameters of cigarette filters, none of these are able to quantify the visual characteristics of the ends of tubular, and related, filters.